docs: def calc_across_paths_textnodes(paths_nodes, dbg=False):
      """Given a list of parent paths tupled with children textnodes, plus
      initialized feature values, we calculate the total and average string
      length of the parent's children textnodes.
      """
  
      # for (path, [textnodes],
      #           num. of tnodes,
      #           ttl strlen across tnodes,
      #           avg strlen across tnodes.])
      for path_nodes in paths_nodes:
          cnt = len(path_nodes[1][0])
          ttl = sum([len(s) for s in paths_nodes[1][0]])  # calculate total
          path_nodes[1][1] = cnt                          # cardinality
          path_nodes[1][2] = ttl                          # total
          path_nodes[1][3] = ttl/ cnt                     # average
          if dbg:
              print(path_nodes[1])